In many fluid dispensing applications, it is desirable to control the flow of a fluid precisely and to have the ability to change flow control parameters rapidly. Such control may be desired either to maintain a given flow rate in the face of perturbations such as changes in the fluid's flow characteristics or supply pressure or, to effect similarly rapid changes in the flow rate such as may be required to account for changes in the relative speed between the dispenser and a workpiece onto which the fluid is being dispensed.
When dispensing viscous fluids such as certain lubricants, adhesives, sealants and the like, it is often necessary to apply the material to the surface of a workpiece in a bead or other form of deposit containing a desired amount of material per unit length. In high production processes or where the deposit of material must be positioned with accuracy, robot arms are often used to apply the material by rapidly guiding a dispensing nozzle in a programmed pattern over the surface of the workpiece. Depending on the application, the fluid being dispensed may either be projected some distance from the nozzle in a high velocity stream or extruded from the nozzle at lower velocity with the nozzle located closer to the workpiece. In either case, the amount of material applied per unit of lineal distance along the deposit will tend to vary according to both the flow rate of material discharged from the dispensing nozzle and the speed of the nozzle with respect to the workpiece.
In the automotive industry, such a process is used to apply a bead of sealant around the periphery of the inside surface of automobile doors before joining the inside panel to the door. Along long, straight portions of the pattern, a robot arm can move the nozzle quickly. However, where the desired bead pattern changes direction abruptly, such as around the corners of a door panel, the robot arm must be slowed down to achieve a required bead positioning accuracy. It can be appreciated that if the flow rate of the dispensed fluid material is held fixed, the amount of material in the applied bead will increase as the robot arm is decelerated to negotiate changes in direction and will decrease as the robot arm is accelerated. Likewise, changes in the fluid supply pressure or changes in the viscosity of the fluid material will tend to disrupt control over the size of the bead.
An apparatus and method which effectively addresses these difficulties is fully described in co-pending, commonly assigned U.S. patent application Ser. No. 06/924,940 now abandoned which is expressly incorporated herein by reference. That application discloses, inter alia, a fluid dispensing method and apparatus wherein a servo actuator drives a substantially infinitely variable fluid metering valve located in close proximity to a fluid discharge nozzle. The metering valve may suitably comprise a needle valve having a valve seat and a stem moveable relative the seat to vary the flow through the seat. A pressure sensor at the nozzle generates a signal correlated to the instantaneous flow rate of the dispensed fluid. Control over liquid flow rate is achieved by connecting the dispenser in a closed-loop system in which the servo actuator is driven by a control current derived in accordance with the difference between the flow rate signal and a driving signal representing a desired flow rate. In robotic applications, the driving signal is preferably related to a toolspeed signal generated by the controller of the robot carrying the dispenser so that the control current will vary as required to maintain a uniform bead of fluid material even during relatively rapid changes in the relative speed between the dispenser and the workpiece onto which material is dispensed.
Co-pending commonly assigned U.S. patent application Ser. No. 07/164,536 now U.S. Pat. No. 4,842,162 which is also expressly incorporated herein by reference in its entirety discloses a fluid dispensing method and apparatus wherein control of the actuator which drives the metering valve of a dispenser is improved by using a feedback signal which varies according to both the relative position and the relative velocity of the stem of the metering valve with respect to the seat of the metering valve. In a preferred embodiment, such a position-dependent velocity signal is generated by a transducer including both a magnet and a coil. The magnet and coil are juxtaposed to be moved relative one another in a manner tracking the relative movement between the stem and seat of the metering valve so that the magnitude of the position-dependent velocity signal increases as the relative velocity of the valve stem with respect to its seat increases. Also, for any given velocity, the influence of the magnet on the coil is such that the magnitude of the signal increases as the distance between the stem and seat decreases. By using such a position-dependent velocity signal as a feedback signal in the control loop operating the metering valve, rapid yet highly stable response can be achieved.
While the foregoing inventions provide dispensing apparati and methods which can be used to effect quite accurate control over the amount of fluid material per unit length contained in a deposit of material formed on a workpiece, it is sometimes also desirable to have the ability to exercise control over the conformation of the deposit. As used herein and in the claims, the term "conformation" refers to the shape of the bead in terms of its cross-sectional profile and the formation of its surface. For example, in some cases it may be desired to form a bead of material having a smooth surface and a high, well rounded profile while at other times a wider, flatter bead having a rippled surface may be desired. Moreover, it may be necessary or desirable to provide beads having differing conformations on different areas of the same workpiece with such beads being either mutually spaced or contiguous with one another. In the case of a spray or spatter type pattern not in the form of a continuous bead, "conformation" refers to such characteristics as the width of the pattern, the spacing, size and/or shape characteristics of the liquid as deposited on the workpiece.
Of course, the geometry of the outlet of the dispensing nozzle has an effect on the conformation of the deposited liquid as does the pressure under which the fluid is discharged from the nozzle outlet. However, it is not practical to attempt to vary the fluid pressure at the nozzle to effect significant changes in conformation of the deposited liquid material since such pressure variations would frustrate the crucial objective of accurately controlling the rate at which the material is dispensed. Substitution of nozzles is also not an attractive alternative since either a changeover delay or a more complex multi-nozzle dispensing apparatus would be required. The additional bulk and mass of such apparatus would limit its maneuverability and increase the weight supporting demand on manipulating robots. Moreover, changes in nozzle geometry alone may not be sufficient to provide a bead or other form of deposit of liquid material which contains a desired amount of material per unit length and also exhibits a particular desired and substantially uniform conformation.
Accordingly, there is a need for a simple and efficient apparatus and method for dispensing liquid material onto a workpiece in a deposit having a conformation substantially different from that which would normally be produced by a given nozzle at a given pressure and which is capable of maintaining a given conformation despite variations in the liquid flow rate and/or changes in the relative speed between the dispenser and the workpiece. There further exists a need for such an apparatus and method capable of selectively varying said conformation quickly and predictably. There is also a need for such an apparatus and method which avoids such undesired disruptions of said conformation as might otherwise be induced due to momentary irregularities in liquid supply pressure such as those which may be associated with the intake phase of the pumping cycle of a pump supplying liquid material to a dispenser.